The invention relates to an apparatus and method for measuring repeatedly the absorption of electro-magnetic radiation by a plurality of specimens during a period of time. More particularly this invention concerns an apparatus and method in which each of a plurality of fluid samples or aliquots (portions of the samples) in reaction vessels or cuvettes is subjected to chemical reaction with different reagents. The absorbance of each aliquot repeatedly is determined during the reaction period. The measurement herein involves accurately ascertaining the value of electromagnetic radiation absorption at a particular wavelength by the fluids in the cuvettes and converting the signals from an analog to a digital form so that digital transmission, processing, storage and manipulation may occur.
It is desirable to make such an analysis on a continuous process in which the apparatus continues to operate as long as there are samples to be tested, the old samples and their tested aliquots being replaced by new samples and their aliquots without interruption of the operation of the testing apparatus. Such continuous operation includes one or more photometric measurements on a given aliquot by one or more photometers. It is preferably that the analog transmittance signals received each time a reaction vessel passes through a light beam be converted into digital absorbance signal.
The continuous analyzers of interest typically supply sample portions to the reaction vessels which are monitored by measuring the absorption or transmission of light by the fluids in the cuvettes at a particular wavelength or wavelengths. Sample fluids placed in cuvettes typically are body fluids of a specific patient with one or more tests related to the patient's condition of health being conducted. It is therefore critical that the signals obtained from the fluids in the cuvettes be both accurate and repeatable. The sampling of the peak absorbance signal of each aliquot should be precisely repeatable for each cuvette and each light beam passing through the cuvette.
This application is an improvement over the application Ser. No. 177,092, filed Aug. 11, 1980, now pending. In the prior application the signals derived from the photometer beams passing through the cuvettes must be optimized, WOW in a rotating photometer carrier must be compensated for and the base line of the sample signal established during the dark or non-sample condition must be compensated for. Moreover, a problem presented by measuring the value of the light transmitted by the cuvette and sample fluids contained therein is how to convert the low level millivolt signal which has microvolt accuracy to digital absorbance data having an accuracy of plus or minus one digit with the fixed mechanical limitations of such a device.
The invention herein overcomes these and other disadvantages to provide reliable, accurate and repeatable information on a continuous basis capable of being processed in a high speed digital processor.